(a) What is co-firing?

Co-firing refers to the technology whereby biomass is fired with fossil fuels at thermal power stations and so on. The advantage of this technology is that, simply by making some minor revisions to existing equipment to enable biomass treatment, it becomes possible to fire biomass in highly efficient large-scale combustion facilities. Here, an introduction is given to research and development of coal and woody biomass co-firing technology that has been jointly implemented by The Chugoku Electric Power Co., Inc., Hitachi Ltd. and Babcock-Hitachi K. K.

(b) Goals

Upon burning around 5-10% of woody biomass in a coal fired power station (hereafter the proportion of mixed fuel is shown in terms of the calorific base), they aimed to secure stable operation and to clear environmental standards while at the same time minimizing any reduction in generating efficiency. Aiming for generating efficiency equivalent to approximately 40% the level in the existing coal fired power station, they held the reduction in efficiency at the sending end to within 0.5% when the mixed fuel firing rate was 5% (and within 0.8% when the mixed fuel firing rate was 10%).

(c) Feedstock

Based on component analysis of pine, cedar, cypress and bamboo, etc., woody biomass has greater volatile content than coal (bituminous coal), its fuel ratio (ratio of fixed carbon content to volatile matter content) is approximately 1/10 of that of coal, and its ash content is low. Upon conducting tests in a small hammer mill, etc., grinding power consumption for woody biomass was at least 10 times higher than that for the same weight of coal and simultaneous grinding testing of coal and woody biomass revealed that the grindability of coal is reduced a lot when the mixing ratio of wood is increased.

(d) Process flow

Figure 4.1.2 shows the flow diagram of the biomass pretreatment equipment. The woody biomass consisted of thinned timber and bamboo chips procured from the Chugoku region with a size of no more than 50 mm and water content of 50wt%. The chips were crushed to a size suitable for drying (no more than 20 mm) and dried to a water content of 20% or less. Next, two types of pulverizer were combined to regulate the pieces to a top size of 1~5 mm, and the materials were blown into the furnace by fixed quantity feeder. The mixed fuel firing rate was a maximum of 15%. Two types of burner were used: a coaxial coal and biomass mixed burner and a biomass dedicated burner (separately installed).

(e) Results

Figure 4.1.3 shows some of the test results. As the proportion of mixed fuel was increased, the unburned matter and relative value of NOx fell in both the coaxial burner and separately installed burner. The reduction in the unburned matter indicates that mixing of the highly volatile biomass caused the ambient temperature to rise and the combustion efficiency of the fuel itself to increase. The rate of decline in NOx was lower than the value forecast from the rate of decline iel.

(f) Efficiency

Based on the results of the combustion tests, the basic equipment and system composition in the case of application to the existing coal fired power station (selecting three units of 75-500 MW) was examined and the power generating efficiency and generating cost were assessed. Factors that cause the mixed fuel firing to affect power generating efficiency are thought to be changes in boiler efficiency and auxiliary power. In the case where thinned wood chips having water content of 30% and chip size of 50 mm or less following natural drying of the forest land, etc. are introduced to the power station, this wood is converted to crushed biomass of 20% water content and 2-mm particle size following pulverization and conveyance, and this is combusted in the boiler, whereas the unburned matter of ash is lower than in the case of firing only coal, the boiler efficiency drops slightly due to the water content of the woody biomass. As for auxiliary power of the plant, from the pilot test findings and based on estimation of the crushing power in the case where a two-stage shock crusher is used, the drop in overall sending end efficiency is 0.44% and 0.77% when the proportion of mixed fuel is 5% and 10% respectively, and these values are within the respective targets of 0.5% and 0.8%. Furthermore, upon comparing costs between woody biomass-only firing (10 MW) and mixed fuel firing, since the mixed fuel firing was less expensive than dedicated firing (11.3 yen/kWh), the superiority of mixed fuel firing was confirmed.